Chapter 11- cell-cell interactions

Are cells self-contained?

No, they must interact with other cells and the environment. Even single-celled organisms aren't

What's outside the membrane?

Extracellular matrix (ECM), cell walls

Extracellular Matrix (ECM)

The "stuff" found outside of the cell in animal tissues made of collagen and proteoglycans which supports the cell, segregates tissues, and regulates intercellular communication

cell walls

Cellulose surrounding the cells of plants (and bacteria, fungi, and some protists) which protect the cell, maintain its shape, and prevent excess water uptake

Tight junctions

Form water tight seal between cells so no extracellular fluid leaks; restricts passive movement

Desmosomes

Bind the cytoskeletons of adjacent cells together. Cell-cell adhesions are selective

Cadherins

Proteins in desmosomes that link cells. Type varies between tissues

gap junctions (animals)

Membrane proteins line up to form channels between cells; allows passage of ions and small molecules which allows cells to communicate with each other

Plasmodesmata (plants)

membrane-lined channels that go through cell walls connect adjacent cells; lets cells communicate with nearby cells

Long distance signaling

- Uses hormones (endocrine signaling), travels through bloodstream to reach target cells. Ability of cell to respond depends on whether it has receptors that can bind the signaling molecule

Three stages of cell signaling

reception, transduction, response

Reception

A receptor protein on the surface of the cell (or inside) binds the signaling molecule (ligand) and transmits the signal. The binding is very specific and causes the receptor to change its shape

G protein-coupled receptors (GPCRs)

plasma membrane receptors that work with the help of a G protein. The G protein acts as an on/off switch. When the GTP is bound it's on, when the GDP is bound it's off. The active G protein moves from the receptor to the enzyme. Binding to the enzyme often activates the production of secondary messengers. GTPase domain enzyme shuts the reaction off by cleaving off a phosphate from GTP and returning it to the inactive form GDP so it's ready to receive the next signal

Receptor Tyrosine Kinases (RTKs)

Signaling molecule binds to RTK and makes it combine with another RTK. This causes ATP to give up a phosphate to GDP, making GTP and becoming ADP itself. This activates Ras. The activated Ras triggers another phosphorylation where ATP gives a phosphate to an inactive kinase, activating it and causing a phosphorylation cascade where each kinase phosphorylation the next kinase until a response is triggered in the cell.

abnormally activated RTKs are associated with

many types of cancer

Ion channel receptors

Binding of ligand causes conformational change, allowing the gate to open and let specific ions pass through and send signals. Once the ligand leaves, the gate closes again

intracellular receptors

Signaling molecule moves straight through the membrane and binds to a receptor in the cytoplasm which is transported to the nucleus and triggers a change in gene expression

Transduction

The relay of signals from receptors to target molecules in the cell. This often involves several steps which allows the signal to be amplified (gets a larger response) and allows for more regulation. This also activates enzymes to produce second messengers and activates phosphorylation cascade. The message is passed along a change in protein conformation (proteins change shape as they have phosphate groups attached and removed)

Protein kinase

An enzyme that transfers phosphate groups to a protein, turning it on

dephosphorylation

removal of a phosphate group, turning a protein off

secondary messengers

small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion and spread a message

cyclic adenosine monophosphate (cAMP)

A secondary messenger. A G protein is activated by a signaling molecule and then activates adenylyl Cyclase which turns ATP into cAMP which then activates protein kinase A which causes a cellular response

Ca2+ as a second messenger

Used as a secondary messenger in both G-protein and tyrosine kinase receptor pathways. Calcium goes into the cell and activates various proteins which causes a cellular response. Increasing Ca2+ concentration causes muscle contraction, secretion, and cell division.

Is levels of calcium higher inside or outside the cell

10000x higher outside the cell. Calcium is actively transported out of the cell or into the ER so that there can be a gradient available when a message needs to be spread

Response

Change in gene expression; regulation of the activity of a protein

Nuclear response

Slower response that takes place in a cell which activates certain gene. Initial signal molecule (growth factor) triggers phosphorylation cascade, last kinase of sequence enters nucleus and activates gene-regulating protein (transcription factor). Transcription factor binds to DNA producing mRNA which makes a protein

Cytoplasmic response

Signaling pathway controls the activity of proteins already present in the cell and does not directly affect transcription of genes

Differential gene expression

the expression of different genes by cells with the same genome

Crosstalk

multiple pathways interact to modify response

Termination of the signal

To respond to new signals, old signals must be shut off. Binding of signaling molecules to receptors is reversible, the messengers return to their inactive form. GTP hydrolyzes to GDP, cAMP is converted to AMP, phosphatases remove phosphates to deactivate proteins

Signaling in unicellular organisms

The process of reception and signal transduction is very similar between unicellular and multicellular organisms. Often relay information environment

quorum sensing

the regulation of gene expression in response to fluctuations in cell-population density. Changes the behavior of the organism. Unicellular organisms can come together and act multicellular